In the context of the present application, the term turbid medium is to be understood to mean a substance consisting of a material having a high light scattering coefficient, such as for example intralipid solution or biological tissue. Further, light is to be understood to mean electromagnetic radiation, in particular electromagnetic radiation having a wavelength in the range from 180 nm to 1400 nm. The term “optical properties” covers the reduced scattering coefficient μ′s and the absorption coefficient μa. Furthermore, “matching optical properties” is to be understood as having a similar reduced scattering coefficient μ′s and a similar absorption coefficient μa.
A method for imaging the interior of turbid media, e.g. for breast cancer screening, which has become popular in recent years is imaging by use of light, in particular using light in the near infrared (NIR). Such methods are implemented in mammography devices and devices for examining other parts of human or animal bodies. A prominent example for such a method for imaging the interior of a turbid medium by means of light is Diffuse Optical Tomography (DOT). For example, such a DOT device for imaging the interior of a turbid medium uses a light source to irradiate the turbid medium and photodetectors for measuring a part of the light transported through the turbid medium, i.e. its intensity. A control unit is provided for controlling the scanning process. A processing unit is provided for reconstructing an image of the interior of the turbid medium on the basis of the measured intensities. Some of the known devices are particularly adapted for examining female breasts. In order to allow the examination of the turbid medium, the device is provided with a receiving portion enclosing a measurement volume and arranged to receive the turbid medium. Light from the light source is coupled into the receiving volume and into the turbid medium. The light is chosen such that it is capable of propagating through the turbid medium. For imaging an interior of a female breast, light in the NIR (near infrared) is typically used. Scattered light emanating from the turbid medium as a result of coupling light into the receiving volume is coupled out of the receiving volume. Light coupled out of the receiving volume is used to reconstruct an image of an interior of the turbid medium. Due to different sizes of the turbid media to be examined, the size of the receiving portion may not perfectly match the size of the turbid medium, i.e. a space remains between the boundary of the receiving volume and the turbid medium. The part of the turbid medium under investigation is surrounded by a scattering medium (coupling medium) filling the space in the receiving volume. The scattering medium is chosen such that the optical parameters of the scattering medium, such as the absorption and scattering coefficients, are similar to the corresponding optical parameters of the turbid medium. The light source subsequently irradiates the turbid medium from different directions and the photodetectors measure a part of the light transmitted through the turbid medium. A plurality of such measurements are performed with the light directed to the turbid medium from different directions and, based on the results of the measurements, i.e. the obtained data set, the processing unit reconstructs the image of the examined turbid medium.
According to one development of this method, attenuation scans for light are performed in which the attenuation of light is detected for a plurality of combinations of source positions and detection positions. In these measurements the intrinsic contrast of the turbid medium is used, i.e. light at different wavelengths is attenuated by different amounts due to the presence of scatterers and chromophores such as oxy-hemoglobin, deoxy-hemoglobin, water, and lipids. From these attenuation scans, absorption images of the turbid medium can be reconstructed as well as images of physiological parameters such as e.g. the hemoglobin concentration. This technology has become known as Diffuse Optical Tomography (DOT).
According to a further development of this method, a fluorescent contrast agent which preferentially accumulates at lesions in the turbid medium under investigation, e.g. cancerous tissue in a female breast, is administered for the measurement. The turbid medium is irradiated with light from a light source, preferably a laser, and the fluorescent light which is emitted by the turbid medium is detected. From this measurement, a volumetric image of the fluorescence emission by the breast is reconstructed, i.e. exogenous contrast is used. Thus, the spatial distribution of the contrast agent in the turbid medium is reconstructed. This method is called Diffuse Optical Fluorescence Tomography.
Devices have been developed which are adapted to perform both attenuation measurements and fluorescence measurements. In such devices, attenuation measurements are performed for a plurality of wavelengths in a certain range of wavelengths. The data obtained in the attenuation measurements allows reconstructing a spatially resolved image of the absorption properties of the turbid medium for the plurality of wavelengths. Thus, it can be reconstructed from the attenuation measurements how the light travels through the turbid medium at these wavelengths. In order to perform fluorescence measurements, the fluorescent contrast agent is explicitly excited and solely the light emitted by the fluorescent contrast agent is measured. This is for example achieved by introducing appropriate filters in the light paths between the measurement volume and a detection unit. However, the light emitted by the fluorescent contrast agent comprises a band of wavelengths located in a range of wavelengths which can differ from the wavelengths probed by the attenuation measurements. Thus, for the relevant wavelengths of the fluorescence measurement, there arises the problem that it is not known how the light interacts with the turbid medium.
In a method known to the applicant, the approximation is used that the interaction of light with the turbid medium is substantially the same for the wavelengths relevant for the attenuation measurements and for the range of wavelengths relevant for the fluorescence measurement. However, this approximation does not hold in general and the quality of this approximation differs depending on e.g. different patients and the constitution of their tissue. As a result, current methods for reconstructing fluorescence images of the interior of turbid media, i.e. for reconstructing an image of the spatial distribution of the fluorescent contrast in the turbid medium, comprise the problem that concentrations of the fluorescent contrast agent at different positions in the turbid medium cannot be determined to a satisfactory degree from the reconstructed image.